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PubMed1979

Nonlinear spatial summation and the contrast gain control of cat retinal ganglion cells.

1. We studied how responses to visual stimuli at spatially separated locations were combined by cat retinal ganglion cells. 2. The temporal signal which modulated the stimuli was a sum of sinusoids. Fourier analysis of the ganglion cell impulse train yielded first order responses at the modulation frequencies, and second order responses at sums and differences of the input frequencies. 3. Spatial stimuli were spots in the centre and periphery of the cell's receptive field. Four conditions of stimulation were used: centre alone, periphery alone, centre and periphery in phase, centre and periphery out of phase. 4. The effective first order response of the centre was defined as the response due to centre stimulation in the presence of periphery stimulation, but independent of the relative phases of the two regions. Likewise, the effective first order response of the periphery was defined as the response due to periphery in the presence of centre stimulation, but independent of the relative phases of the two regions. These effective responses may be calculated by addition and subtraction of the measured responses to the combined stimuli. 5. There was a consistent difference between the first order frequency kernal of the effective centre and the first order kernel of the centre alone. The amplitudes of the effective centre responses were diminished at low frequencies of modulation compared to the isolated centre responses. Also, the phase of the effective centre's response to high frequencies was advanced. Such non-linear interaction occurred in all ganglion cells, X or Y, but the effects were larger in Y cells. 6. In addition to spatially uniform stimuli in the periphery, spatial grating patterns were also used. These peripheral gratings affected the first order kernal of the centre even though the peripheral gratings produced no first order responses by themselves. 7. The temporal properties of the non-linear interaction of centre and periphery were probed by modulation in the periphery with single sinusoids. The most effective temporal frequencies for producing non-linear summation were: (a) 4-15 Hz when all the visual stimuli were spatially uniform, (b) 2-8 Hz when spatial grating patterns were used in the periphery. 8. The characteristics of non-linear spatial summation observed in these experiments are explained by the properties of the contrast gain control mechanism which we have previously postulated.

Action Potentials↗
PubMed1981

Neural mechanisms of spatial tactile discrimination: neural patterns evoked by braille-like dot patterns in the monkey.

1. The experiments reported here were designed to investigate the responses of cutaneous mechanoreceptive afferents to spatially configured dot patterns scanned across the skin. Braille-like patterns were selected because the discrimination of Braille characters must depend on spatial patterning rather than some other facet of the afferent discharge. 2. A multifactorial experimental design was used in which each afferent fibre was studied using every combination of six dot patterns, two dot sizes, two dot spacings, two contact forces and two scanning velocities. Two other factors, scanning direction relative to the skin ridges and intermittent versus continuous scanning, were studied. 3. Beside the general question concerning the response properties of the mechanoreceptive afferents, three major questions were addressed here. (i) What is the critical spatial dimension at which neural spatial patterning breaks down and below which tactual discrimination must depend on facets of the afferent discharge other than spatial neural patterning? (ii) Which mechanoreceptive population sets this critical dimension? (iii) Why is tactual discrimination enhanced by lateral scanning? 4. The results presented here suggest that the critical dimension, below which spatial neural patterning breaks down, is of the order of 1.0 mm and that the slowly adapting (SA) afferent fibres are responsible for this limit. 5. At dimensions above approximately 1.0 mm the spatial contrast between peaks and troughs in the SA discharge is markedly enhanced during scanning. When the skin is stationary the discharge rates in the SA population drop rapidly to low levels. A second possible reason for enhanced tactual discrimination during scanning is related to the increased spatiotemporal information in a coherent pattern of neural activity moving across a discrete population of afferent fibres. 6. The effects of variations in conduction velocity are analysed and it is shown that they place serious constraints on the transmission of spatiotemporal information.

Animals↗
PubMed1983

Spatial properties of cells in the rabbit's striate cortex.

The rabbit's striate cortex contains a variety of receptive-field types, including concentric, uniform, simple and complex types. The spatial summation of these receptive fields was studied. Two types of linearity tests were performed: null tests and drifting grating tests. For the null tests, stimuli were sine-wave gratings, temporally modulated by either a 1 Hz sine-wave or by a 1 Hz on-off square wave. For the drifting grating tests, the stimuli were sine-wave gratings moved across the cells' receptive fields at 1-3 Hz. Fourier analyses were performed on the averaged data from both the null tests (sine-wave modulation) and the drifting grating tests. Thirty-one concentric cells were studied. Of these, nine were classified as X and fourteen as Y based upon their responses to ratings. The X cells exhibited linear spatial summation; their responses were mainly at the fundamental modulation frequency. The Y cells exhibited non-linear spatial summation. Their responses contained, in addition to a fundamental component, a second harmonic component whose relative strength increased as the spatial frequency of the display was raised. Eight other concentric cells responded poorly to gratings and were classified as sluggish. Other properties of concentric cells were examined, particularly their responses to standing contrast. The X cells' responses were generally sustained, the Y cells' responses generally transient. Sluggish cells were either sustained or transient, but their responses to standing contrast could frequently be distinguished from those of X and Y cells. All five uniform cells studied exhibited non-linear spatial summation. Their behaviour on the linearity tests was indistinguishable from that of Y cells. Of the seventeen simple cells studied, seven behaved like X cells (linear simple) and ten like Y cells (non-linear simple). However, near their optimum spatial frequencies the responses of both linear and non-linear simple cells were dominated by the fundamental component. Two complex cells behaved like complex cells in the cat's striate cortex (Movshon, Thompson & Tolhurst, 1978b). Their responses were qualitatively like those of non-linear simple cells, but were quantitatively more non-linear. Near their optimum spatial frequencies, harmonic components dominated their responses. The behaviour of nine other complex cells was difficult to describe. Generally, they were unresponsive to gratings. When they could be made to respond to gratings, they gave phase-sensitive responses like those of simple cells.

Animals↗
PubMed1996

Spatially non-uniform Ca2+ signals induced by the reduction of transverse tubules in citrate-loaded guinea-pig ventricular myocytes in culture.

1. Ratiometric confocal microscopy and the whole-cell patch clamp technique were used to simultaneously record intracellular Ca2+ transients and membrane currents from guinea-pig ventricular myocytes. Intracellular dialysis with the low-affinity Ca2+ buffer citrate enabled us to record and analyse Ca2+ transients caused by Ca2+ influx alone and by additional Ca2+ release from the sarcoplasmic reticulum (SR) in the same cell. 2. In freshly isolated adult myocytes (used within 1-4 h of isolation) both types of Ca2+ transients ('Ca2+ entry' and 'Ca2+ release' transients) were spatially uniform regardless of the Ca2+ current (ICa) duration. In contrast, Ca2+ transients in short-term cultured (1-2 days) myocytes exhibited marked spatial inhomogeneities. ICa frequently evoked Ca2+ waves that propagated from either or both ends of the cardiac myocyte. Reduction of the ICa duration caused Ca2+ release that was restricted to one of the two halves of the cell. 3. Analysis of the Ca2+ entry signals in freshly isolated and short-term cultured myocytes indicated that the spatial properties of the Ca2+ influx signal were responsible for the spatial properties of the triggered Ca2+ release from the SR. In freshly isolated ventricular myocytes Ca2+ influx was homogeneous while in short-term cultured cells pronounced Ca2+ gradients could be found during Ca2+ influx. Spatial non-uniformities in the amplitude of local Ca2+ entry transients were likely to cause subcellularly restricted Ca2+ release. 4. The changes in the spatial properties of depolarization-induced Cai2+ signals during short-term culture were paralleled by a decrease (to 65%) in the total cell capacitance. In addition, staining the sarcolemma with the membrane-selective dye Di-8-ANEPPS revealed that, in cultured myocytes, t-tubular membrane connected functionally to the surface membrane was reduced or absent. 5. These results demonstrate that the short-term culture of adult ventricular myocytes results in the concomitant loss of functionally connected t-tubular membrane. The lack of the t-tubular system subsequently caused spatially non-uniform SR Ca2+ release. Evidence is presented to show that in ventricular myocytes lacking t-tubules non-uniform SR Ca2+ release was, most probably, the result of inhomogeneous Ca2+ entry during ICa. These findings directly demonstrate the functional importance of the t-tubular network for uniform ventricular Ca2+ signalling.

Animals↗
PubMed2004

Correlation of local and global orientation and spatial frequency tuning in macaque V1.

Visual cortical neurones display a variety of visual properties. Among those that emerge in the primary visual cortex V1 are sharpening of selectivity for spatial frequency and for orientation. The selectivity for these stimulus attributes can be measured around the peak of the tuning function, usually as bandwidth. Other selectivity measures take into account the response across a broader range of stimulus values. An example of such a global measure is the circular variance of orientation tuning. Here we introduce a similar measure in the spatial frequency domain that takes into account the shape of the tuning curve at frequencies lower than the peak, called the low-spatial frequency variance. Our recent studies with dynamic stimuli suggest that the selectivity for spatial frequency and orientation is strongly correlated with the degree of suppression at low spatial frequencies and off-axis orientations. Here we extend the study of the global tuning to stimulus conditions that measure the response of cells to the presentation of drifting sinusoidal grating stimuli for periods of a few seconds. We find that under such steady-state stimulus conditions there is a strong correlation between the global selectivity measures, orientation circular variance and low spatial frequency variance. Consistent with previous studies, there is a weaker correlation between the local tuning measures, orientation and spatial frequency bandwidth. These results support the idea that there are multiple factors that contribute to tuning and that suppression observed in dynamic experiments is also likely to contribute to the global selectivity for steady-state stimuli.

Animals↗
PubMed1994

Analysis of the spatial-frequency-dependent DQE of optically coupled digital mammography detectors.

The effect of optical coupling efficiency on the spatial-frequency-dependent propagation of signal and noise is considered for x-ray image detectors for digital mammography in which a phosphor screen is optically coupled to a charge-coupled device (CCD) image array. For experimental purposes, optical coupling between a Gd2O2S:Tb phosphor screen and a CCD image array was provided by relay lenses. Neutral density filters were inserted between the lenses to vary the optical coupling efficiency without altering the inherent spatial resolution. The total coupling efficiency, defined as the number of electrons (e-) recorded in the CCD per x-ray interaction in the phosphor, was calculated in each case. The modulation transfer function, and the contributions to the total noise power spectrum (NPS) of x-ray quantum noise, secondary quantum noise, and inherent detector noise were measured as a function of coupling efficiency. These data were used to calculate the spatial-frequency-dependent detective quantum efficiency [DQE(f)]. The NPS due to x-ray quantum noise had a significant spatial-frequency dependence for coupling efficiencies of more than 9 e- per x-ray interaction, but little spatial-frequency dependence for coupling efficiencies of less than 2 e- per x-ray interaction. These results indicate that to preserve high spatial-frequency values of DQE(f), and to ensure that images are x-ray quantum-noise limited at high spatial frequencies, a coupling efficiency on the order of 10 e- per x-ray interaction is required.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrons↗
PubMed2006

The influence of spatial separation on divided listening.

If spatial attention acts like a "spotlight," focusing on one location and excluding others, it may be advantageous to have all targets of interest within the same spatial region. This hypothesis was explored using a task where listeners reported keywords from two simultaneous talkers. In Experiment 1, the two talkers were placed symmetrically about the frontal midline with various angular separations. While there was a small performance improvement for moderate separations, the improvement decreased for larger separations. However, the dependency of the relative talker intensities on spatial configuration accounted for these effects. Experiment 2 tested whether spatial separation improved the intelligibility of each source, an effect that could counteract any degradation in performance as sources fell outside the spatial spotlight of attention. In this experiment, intelligibility of individual sources was equalized across configurations by adding masking noise. Under these conditions, the cost of divided listening (the drop in performance when reporting both messages compared to reporting just one) was smaller when the spatial separation was small. These results suggest that spatial separation enhances the intelligibility of individual sources in a competing pair but increases the cost associated with having to process both sources simultaneously, consistent with the attentional spotlight hypothesis.

Acoustic Stimulation↗
PubMed1989

Spatial selectivity of rat hippocampal neurons: dependence on preparedness for movement.

The mammalian hippocampal formation appears to play a major role in the generation of internal representations of spatial relationships. In rats, this role is reflected in the spatially selective discharge of hippocampal pyramidal cells. The principal metric for coding spatial relationships might be the organism's own movements in space, that is, the spatial relationship between two locations is coded in terms of the movements executed in getting from one to the other. Thus, information from the motor programming systems (or "motor set") may contribute to coding of spatial location by hippocampal neurons. Spatially selective discharge of hippocampal neurons was abolished under conditions of restraint in which the animal had learned that locomotion was impossible. Therefore, hippocampal neuronal activity may reflect the association of movements with their spatial consequences.

Animals↗
PubMed1997

Spatial Patterns of Rhizoplane Populations of Pseudomonas fluorescens.

Geostatistical analysis was used to compare rhizoplane colonization patterns of an antibiotic-producing biological control bacterium versus a non-antibiotic-producing mutant strain. Pea seeds were inoculated with Pseudomonas fluorescens 2-79RN(inf10) or P. fluorescens 2-79-B46 (a phenazine-deficient Tn5 mutant of P. fluorescens 2-79RN(inf10)) (10(sup8) CFU/pea), planted in sterile sand, and incubated at 20(deg)C. After 3 days, seedlings were prepared for scanning electron microscopy. Photomicrographs (x1,000) of the root surface were taken at the seed-root junction and at 0.5-cm intervals to the root tip. Bacterial counts on the root surface were made in 5- by 5-(mu)m sample units over an area which was 105 by 80 (mu)m. Coordinates and number of bacteria were recorded for each sample unit. Spatial statistics were calculated by covariance for the following directions: omnidirectional, 0, 45, 90, and 135(deg). The ranges of spatial influence and nugget (estimator of spatially dependent variation) were determined. For both P. fluorescens 2-79RN(inf10) and P. fluorescens 2-79-B46, spatial structure was evident along the entire root, particularly in the 0(deg) direction (along the root length) (e.g., range = 24 (mu)m, nugget = 0.52). The degree of spatial dependence observed indicated aggregation of bacterial cells. No differences were detected in the spatial patterns of colonies of P. fluorescens 2-79RN(inf10) and P. fluorescens 2-79-B46, indicating that the lack of phenazine production did not influence spatial patterns on the rhizoplane.

Journal Article↗
PubMed2004

Spatial analysis and mapping of sexually transmitted diseases to optimise intervention and prevention strategies.

OBJECTIVE: We analysed and mapped the distribution of four reportable sexually transmitted diseases, chlamydial infection/non-gonococcal urethritis (chlamydial infection), gonorrhoea, primary and secondary syphilis (syphilis), and HIV infection, for Wake County, North Carolina, to optimise an intervention. METHODS: We used STD surveillance data reported to Wake County, for the year 2000 to analyse and map STD rates. STD rates were mathematically represented as a spatial random field. We analysed spatial variability by calculating and modelling covariance functions of random field theory. Covariances are useful in assessing spatial patterns of disease locally and at a distance. We combined observed STD rates and appropriate covariance models using a geostatistical method called kriging, to predict STD rates and associated prediction errors for a grid covering Wake County. Final disease estimates were interpolated using a spline with tension and mapped to generate a continuous surface of infection. RESULTS: Lower incidence STDs exhibited larger spatial variability and smaller neighbourhoods of influence than higher incidence STDs. Each reported STD had a clustered spatial distribution with one primary core area of infection. Core areas overlapped for all four STDs. CONCLUSIONS: Spatial heterogeneity within STD suggests that STD specific prevention strategies should not be targeted uniformly across Wake County, but rather to core areas. Overlap of core areas among STDs suggests that intervention and prevention strategies can be combined to target multiple STDs effectively. Geostatistical techniques are objective, population level approaches to spatial analysis and mapping that can be used to visualise disease patterns and identify emerging outbreaks.

Adolescent↗
PubMed2005

Temporal gradients in shear, but not spatial gradients, stimulate ERK1/2 activation in human endothelial cells.

We have previously demonstrated temporal gradients in shear stress stimulate endothelial cell proliferation, whereas spatial gradients do not. In the present study, the extracellular signal-regulated kinases 1 and 2 (ERK1/2) pathway was investigated as a possible mediator for the promitogenic effect of temporal gradients. The sudden expansion flow chamber (SEFC) model was used to differentiate the effect of temporal gradients in shear from that of spatial gradients on ERK1/2 activation in human umbilical vein endothelial cells (HUVEC). ERK1/2 activation in the SEFC was not significantly different from control when HUVEC were exposed to spatial gradients alone. When a single temporal impulse was superimposed on spatial gradients, ERK1/2 activation was stimulated 330% (relative to spatial alone) within the region of spatial gradients. Inhibition of the ERK1/2 pathway with U-0126 abolished all effects of temporal gradients. To further separate temporal and spatial gradients, a conventional parallel plate flow chamber was utilized. Acute exposure to oscillations in flow at a frequency of 1 Hz stimulated ERK1/2 activation 620 +/- 88% relative to control, whereas a single impulse of flow increased ERK1/2 activation 166 +/- 19%. Flow without the temporal component did not significantly activate ERK1/2. These results suggest that the ERK1/2 pathway directly mediates the promitogenic effects of temporal gradients in shear stress.

Blood Flow Velocity↗
PubMed2006

Dynamics of spatial resolution of single units in the lateral geniculate nucleus of cat during brief visual stimulation.

Sharpness of vision depends on the resolution of details conveyed by individual neurons in the visual pathway. In the dorsal lateral geniculate nucleus (LGN), the neurons have receptive fields with center-surround organization, and spatial resolution may be measured as the inverse of center size. We studied dynamics of receptive field center size of single LGN neurons during the response to briefly (400-500 ms) presented static light or dark spots. Center size was estimated from a series of spatial summation curves made for successive 5-ms intervals during the stimulation period. The center was wide at the start of the response, but shrank rapidly over 50-100 ms after stimulus onset, whereupon it widened slightly. Thereby, the spatial resolution changed from coarse-to-fine with average peak resolution occurring approximately 70 ms after stimulus onset. The changes in spatial resolution did not follow changes of firing rate; peak firing appeared earlier than the maximal spatial resolution. We suggest that the response initially conveys a strong but spatially coarse message that might have a detection and tune-in function, followed by transient transmission of spatially precise information about the stimulus. Experiments with spots presented inside the maximum but outside the minimum center width suggested a dynamic reduction in number of responding neurons during the stimulation; from many responding neurons initially when the field centers are large to fewer responding neurons as the centers shrink. Thereby, there is a change from coarse-to-fine also in the recruitment of responding neurons during brief static stimulation.

Action Potentials↗
PubMed1987

Visual orientation and spatial frequency discrimination: a comparison of single neurons and behavior.

Neurons in the visual cortex respond selectively to stimulus orientation and spatial frequency. Changes in response amplitudes of these neurons could be the neurophysiological basis of orientation and spatial frequency discrimination. We have estimated the minimum differences in stimulus orientation and spatial frequency that can produce reliable changes in the responses of individual neurons in cat visual cortex. We compare these values with orientation and spatial frequency discrimination thresholds determined behaviorally. Slopes of the tuning functions and response variability determine the minimum orientation and spatial frequency differences that can elicit a reliable response change. These minimum values were obtained from single cells using receiver operating characteristic (ROC) analysis. The average minimum orientation and spatial frequency differences that could be signaled reliably by cells from our sample were 6.4 degrees (n = 22) and 21.3% (n = 18), respectively. These values are approximately 0.20 of the average full tuning width at one-half height of the cells. Although these average values are well above the behaviorally determined thresholds, the most selective cells signaled orientation and frequency differences of 1.84 degrees and 5.25%, respectively. These values are of the same order of magnitude as the behavioral thresholds. We show that, because of slow fluctuations in a cell's responsivity, ROC analysis overestimates response variability. We estimate that these slow response fluctuations elevated our estimates of single cell "thresholds" by, on average, 30%. Our data point to an approximate correspondence between orientation and spatial frequency discrimination "thresholds" determined behaviorally and those estimated from the most selective single cortical cells. Interpretation of this quantitative correspondence is considered in the discussion.

Animals↗
PubMed1988

Binaural neurons in the mustache bat's inferior colliculus. II. Determinants of spatial responses among 60-kHz EI units.

1. The responses to closed-field and free-field sound stimuli were obtained from EI neurons in the 60-kHz isofrequency representation of the mustache bat's inferior colliculus. The goals of the study were 1) to examine how binaural interactions and the directional properties of the ear shape the spatial response of 60-kHz EI neurons and 2) to examine features of the spatial responses of these isofrequency neurons which may encode the location of a sound. 2. The free-field response of monaural neurons was used to describe directional effects of the head and external ears at 60 kHz. Monaural neurons were most sensitive to sounds originating 26 degrees into the contralateral field, with sharply reduced sensitivity in the ipsilateral field and the lateral extreme of the contralateral field. The interaural intensity difference (IID), computed by assuming that the ears had mirror-image directional sensitivities, was greatest for sounds between 26 and 52 degrees off the vertical midline, where IIDs typically ranged from 20 to 30 dB. These values agree well with previous measures of the directional sensitivity of the ear obtained from cochlear microphonic recordings. 3. The response of a monaural unit to closed-field stimulation was used to predict its response to free-field stimulation. This prediction agreed closely with the observed free-field response, indicating that the monaural unit's free-field response could be explained by the directional sensitivity of the contralateral ear and the unit's response as a function of contralateral sound intensity. 4. EI neurons differed in basic features of their binaural response properties, as described in a companion paper. To examine how these features of binaural inhibition shaped spatial responses, we obtained responses to free-field sounds among neurons whose binaural properties had been thoroughly characterized. We then compared the spatial response to a prediction based upon the directional properties of the ear and the sensitivity of the EI neurons to IIDs and intensity. The close agreement between the observed and predicted spatial responses in 11 of 12 EI units indicated that the spatial responses were determined largely by sensitivity to IIDs and intensity and by the directional properties of the ear. 5. EI neurons in our sample were most sensitive, and responded best, to sounds originating 26-39 degrees into the contralateral field. These spatial response features were independent of the binaural properties of EI neurons. Instead, they depended upon the directional properties of the ears at 60 kHz.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗
PubMed1991

Lateral geniculate neurons in behaving primates. III. Response predictions of a channel model with multiple spatial-to-temporal filters.

1. For the experiments reported in these papers, we recorded the responses of lateral geniculate (LGN) neurons to a large set of two-dimensional, black and white patterns based on Walsh functions and to a set of test stimuli. In the first two papers we reported that these neurons encode stimulus-related information in both the strength and the shape of the response waveforms and that there are more than two independent components in the response. These results cannot be explained by existing models. This paper provides a model of LGN neurons that not only accounts for the foregoing observations, but also yields predictions confirmed by direct tests. 2. The model represents a neuron as a set of three parallel channels. The input to each channel is an array of pixel luminances. Each channel consists of an input nonlinearity cascaded into a linear spatial-to-temporal filter. The output of each channel is a basic waveform, a principal component. The response of the neuron is the sum of the outputs of the three channels. 3. The model accounted for much of the variance in the coefficients of the first three principal components of the neuronal responses to the set of Walsh stimuli. Using parameters derived from the responses of neurons to the Walsh stimuli only, the model also predicted the responses to "center-surround" annuli of different contrasts and mean luminances, as well as to superpositions of pairs of Walsh patterns. The model made statistically significant predictions of the coefficients of two of the principal components of these responses. 4. After the parameters of the model had been fit to reproduce the responses of neurons to the Walsh stimuli, we found that the input nonlinearity of the model was compressed at both the high and low luminance levels. This compression produced response saturation that closely resembled the response saturation of neurons reported in the first paper in this series. Although not absolutely smooth, the spatial filter for the first channel had a dominant excitatory or inhibitory center and an antagonistic surround. Thus this spatial filter accounted for both the center and the surround structures of previous models of LGN receptive fields. There was greater variety in the structures of the spatial filters for the second and third channels, but none had a center-surround organization. Many of the spatial filters for these higher channels contained oriented ridges or valleys. Other spatial filters were dominated by a bipolar pair of pixels. 5. The model of LGN neurons that we present in this paper represents an extension over previous models in four ways. First, the model is capable of explaining the responses of neurons to a wider range of luminances than previous models. Second, the model is capable of explaining the shapes of the response waveforms as well as their magnitudes. Third, the concept of a single receptive field is extended to a series of spatial-to-temporal filters. Fourth, the model suggests that LGN neurons provide a description of both the brightness and the form of a stimulus in their response waveforms.

Animals↗
PubMed1994

Population encoding of spatial frequency, orientation, and color in macaque V1.

1. We recorded local field potentials in the parafoveal representation in the primary visual cortex of anesthetized and paralyzed macaque monkeys with a multicontact electrode that provided for sampling of neural activity at 16 sites along a vertical penetration. Differential recordings at adjacent contacts were transformed into an estimate of current source density (CSD), to provide a measure of local neural activity. 2. We used m-sequence stimuli to map the region of visual space that provided input to the recording site. The local field potential recorded in macaque V1 has a population receptive field (PRF) size of approximately 2 deg2. 3. We assessed spatial tuning by the responses to two-dimensional Gaussian noise, spatially filtered to retain power only within one octave. Responses to achromatic band-limited noise stimuli revealed a prominent band-pass spatial tuning in the upper layers, but a more low-pass spatial tuning in lower layers. 4. We assessed orientation tuning by the responses to band-limited noise whose spectrum was further restricted to lie within 45 degrees wedges. The local field potential showed evidence of orientation tuning at most sites. Orientation tuning in upper and lower layers was manifest by systematic variations not only in response size but also in response dynamics. 5. We assessed chromatic tuning by the responses to isotropic band-limited noise modulated in a variety of directions in tristimulus space. Some lower-layer locations showed a nulling of response under near-isoluminant conditions. However, response dynamics in upper and lower layers depended not only on luminance contrast, but also on chromatic inputs. 6. Responses to near-isoluminant stimuli and to low-contrast luminance modulation were shifted to lower spatial frequencies. 7. We determined the extent to which various temporal frequencies in the response conveyed information concerning spatial frequency, orientation, and color under the steady-state conditions used in these studies. In each case, information is distributed in the response dynamics across a broad temporal frequency range, beginning at 4 Hz (the lowest frequency used). For spatial frequency the information rate remains significant up to at least 25 Hz. For orientation tuning and chromatic tuning, the information rate is lower overall and remains significant up to 13 Hz. In contrast, for texture discrimination, information is shifted to lower temporal frequencies.

Animals↗
PubMed1997

Binocular spatial phase tuning characteristics of neurons in the macaque striate cortex.

We employed microelectrode recording techniques to study the sensitivity of individual neurons in the striate cortex of anesthetized and paralyzed monkeys to relative interocular image disparities and to determine the effects of basic stimulus parameters on these cortical binocular interactions. The visual stimuli were drifting sine wave gratings. After the optimal stimulus orientation, spatial frequency, and direction of stimulus movement were found, the cells' disparity tuning characteristics were determined by measuring responses as a function of the relative interocular spatial phase of dichoptic grating pairs. No attempts were made to assess absolute position disparities or horizontal disparities relative to the horopter. The majority (approximately 70%) of simple cells were highly sensitive to interocular spatial phase disparities, particularly neurons with balanced ocular dominances. Simple cells typically demonstrated binocular facilitation at the optimal phase disparity and binocular suppression for disparities 180 degrees away. Fewer complex cells were phase selective (approximately 40%); however, the range of disparity selectivity in phase-sensitive complex cells was comparable with that for simple cells. Binocular interactions in non-phase-sensitive complex cells were evidenced by binocular response amplitudes that differed from responses to monocular stimulation. The degree of disparity tuning was independent of a cell's optimal orientation or the degree of direction tuning. However, disparity-sensitive cells tended to have narrow orientation tuning functions and the degree of disparity tuning was greatest for the optimal stimulus orientations. Rotating the stimulus for one eye 90 degrees from the optimal orientation usually eliminated binocular interactions. The effects of phase disparities on the binocular response amplitude were also greatest at the optimal spatial frequency. Thus a cell's sensitivity to absolute position disparities reflects its spatial tuning characteristics, with cells sensitive to high spatial frequencies being capable of signaling very small changes in image disparity. On the other hand, stimulus contrast had relatively little effect on a cell's disparity tuning, because response saturation occurred at the same contrast level for all relative interocular phase disparities. Thus, as with orientation tuning, a cell's optimal disparity and the degree of disparity selectivity were invariant with contrast. Overall, the results show that sensitivity to interocular spatial phase disparities is a common property of striate neurons. A cell's disparity tuning characteristics appear to largely reflect its monocular receptive field properties and the interocular balance between excitatory and inhibitory inputs. However, distinct functional classes of cortical neurons could not be discriminated on the basis of disparity sensitivity alone.

Action Potentials↗
PubMedDate not supplied

Models of plasticity in spatial auditory processing.

Both psychophysical and physiological studies have examined plasticity of spatial auditory processing. While there is a great deal known about how the system computes basic cues that influence spatial perception, less is known about how these cues are integrated to form spatial percepts and how the auditory system adapts and calibrates in order to maintain accurate spatial perception. After summarizing evidence for plasticity in the spatial auditory pathway, this paper reviews a statistical, decision-theory model of short-term plasticity and a system-level model of the spatial auditory pathway that may help elucidate how long- and short-term experiences influence the computations underlying spatial hearing.

Acoustic Stimulation↗